1. Field of the Invention
The present invention relates to a punch press stripper assembly for use in any application where the withdrawal of the punch causes undesirable deformation of the material in the vicinity of the punch hole. More particularly, the invention involves a mechanical punch stripper that is assembled as a unit onto a punch or removed from a punch without the necessity of time consuming partial disassembly of the punch mechanism. The punch stripper assembly contains an encapsulated preloaded spring element capable of storing a maximum amount of energy with minimum displacement.
2. Description of the Prior Art
The prior art reveals a wide variety of devices for the punching of holes in sheet material and the prevention of sheet deformation surrounding the punched hole upon retraction of the punch from the sheet material. The integrity of the punched hole is preserved by the utilization of a stripper mechanism that is integrally coupled to the punch.
The present invention is an improvement over the punching unit shown and described in U.S. Pat. No. 3,779,113 entitled "Punching Unit" issued Dec. 18, 1973, to Paul Jestin. The above referenced patent shows a combination punch and stripper mechanism that is contained within a vertical bore. The vertical bore is located at the extremity of the upper cantilevered press arm. The vertical bore is coaxially disposed with a bore in the lower press arm for accommodation of a punch die. The punch and stripper mechanism contained in the upper bore can be removed therefrom by a stop detent. After the entire assembly has been removed from the upper press arm, the disengagement of the striker plate permits the punch to be removed from the remainder of the assembly.
In the above described punch and stripper assembly, the stripper subassembly cannot be replaced unless it is released from the press. Only after partial disassembly can the punch be removed.
While the above described punch and stripper assembly may suffice for a single punch application, the assembly removal does not lend itself to those applications where the punch ram utilizes a plurality of punches.
U.S. Pat. No. 3,283,630 entitled "Self-Cleaning Cutting Die with Stripper Element" issued Nov. 8, 1966, to Frank S. Domka shows a combination die and stripper. A single spring is used to bias the tubular stripper sleeve against the workpiece during and subsequent to the actual severing action of the punch. The compression spring is contained within a tubular body element. An adjustable screw threaded end cap coacts with the upper end of the tubular body element, providing a means to pre-load the compression spring. The stripper and the ejector are actuated by the compression spring which is enclosed within the die unit. The die unit itself is compact and may be removed from the platen of a press without disassembly.
U.S. Pat. No. 4,007,653 entitled "Punching Device with Punch Retainer" issued Feb. 15, 1977, to Percy L. Cady describes a punching device that includes a rigid frame for support of the punch assembly. The punch has a threaded upper end for coupling to a combination striker head and compression spring retainer. The compression spring is coaxial with the punch body and is biased against the upper flange of a tubular stripper which is also coaxial with the punch. A secondary spring is biased against the upper flange of the stripper to facilitate its removal from the surface of the workpiece subsequent to the severing of the punched material.
When the punch and stripper unit is removed from the press arm, the tubular stripper would slide off the punch except that a friction device has been positioned between the exterior surface of the cylindrical punch body and the interior surface of the stripper. The friction member is of elastomeric formulation and is positioned within a groove that has been machined in the punch body.
A punch stripper that is frictionally engageable with a punch is shown in U.S. Pat. No. 2,230,043 entitled "Punch Stripper" issued Apr. 22, 1940, to Harry M. Moran. This patent describes a punch stripper that is essentially a barrel shaped rubber spool. The stripper is retained on the punch shank by the friction developed between the cylindrical interior of the stripper and the exterior surface of the punch shank. Although the rubber stripper is removable by sliding it from the punch its application is limited to lighter metal punch applications since the resiliency characteristics are limited and, therefore, this type of stripper does not lend itself to heavy metal punching applications.